Atomizing magnesium



Jan. 18, 1955 N. R. COLBRY ET AL 2,

ATOMIZING MAGNESIUM Filed March 18, 1953 28heets-Sheet l IN V EN TORS.Norman R. Co/bry Gordon FT Hers/7 ey ATTORNEYS Jan. 18 1955 COLBRY ETAL2,699,576

ATOMIZING MAGNESIUM Filed March 18, 1955 2 Sheets-Sheet 2 y INVENTORS.

Norman R. Co/b/y BY Gar-don E He/ s/oey A T TORA/EKS United StatesPatent ()fiice 2,699,576 Patented Jan. 18, 19 55 1 2,699,576 ATOMIZINGMAGNESIUM Norman R. Colin-y, Breckenridge, and Gordon F. Hershey,

Midland, Mich, assignors to The Dow Chemical Company, Midi-and, Mich, acorporation of Delaware Appiication March 18, 1953, Serial No. 343,158 3Claims. (Cl. 1847.2)

The invention relates to methods of atomizing metal. It moreparticularly concerns an improved method of converting molten magnesiuminto fine substantially spherical particles with a minimum of off-sizeparticles.

The two types of methods heretofore proposed of atomizing magnesium arebeset with many dimculties. One of these types of methods involvesimpinging upon a falling stream of the molten magnesium a jet of gaswhich upon striking the molten metal breaks it up into dropletssolidifying as they become cooled in the atmosphere of the jetting gas.A process of this kind is disclosed in U. S. Patents 1,351,865 and2,371,105. The characteristic disadvantages of the method of atomizingmagnesium with a jet of gas are that the atomized particles are notuniform in size; it is difficult, if not impossible, to producerelatively small adequately dust-free particles without screening theproduct many times and reworking the coarser particles; there is alwaysa small proportion of dust-like fines (less than 600 mesh) which aredetrimental in that they are easily flammable, have low corrosionresistance, and such particles cannot be readily separated from the restof the product due to their tendency to cling to the larger particles.Another difficulty in using a jet of gas for atomizing magnesium is thatthe method requires large volumes of an inactive gas which must befiltered and cleaned before recirculation through the process.

The other of the two types of atomizing processes involves impinging themolten magnesium onto a rapidly revolving disc. A method of this type isdisclosed in British patent specification No. 510,320. According to theBritish patent specification, the molten metal to be atomized isimpinged upon a rapidly revolving cooled disc and cooled at the sametime by directing a cooling agent, such as a liquid or gas, against themetal at the point of impingement of the melt upon the disc.

In attempts to atomize molten magnesium by impinging the metal upon acool rapidly revolving steel disc strong enough to withstand thestresses of high speed rotation fail to produce atomized metal. Instead,the metal in part solidifies on and clings to the disc in thick massesfrom which pieces break off from time to time and in part splashes offthe metal already solidified on the disc in relatively large masseswithout atomizing. Cooling the molten metal during the impingement onthe disc accentuates these diificulties. If the disc is heated to atemperature above the melting point of the magnesium so that the metaldoes not solidify on the disc, the disc becomes rapidly eroded. At thesame time, the molten metal leaving the disc as it spins does notatomize into a uniform sized product. Instead, the particlesprogressively become larger and are interspersed with unatomized metalas the operation proceeds and the disc erodes.

Insofar as we are aware, there is no satisfactory method commerciallyavailable by which molten magnesium can be atomized into uniformly sizedfine spherical particles. Accordingly, it is the principal object of theinvention to provide a method which fulfills the foregoing need.

The present invention is predicated upon the discovery that by includingin the magnesium to be atomized from 0.025 to 1.0 per cent of zirconiumand at least 0.25 per cent of zinc, the resulting metal may be atomizedinto fine substantially uniformly sized particles by allowing the moltenmetal to fall freely in a thin stream onto a disc of steel (preferablytool steel) rapidly rotating in a nonreactive gas, the disc beingmaintained at a temperature above the melting point of the molten metal.Under the stated conditions, atomization is effected without splashingof the molten metal and without significant attack upon the disc. Theinvention then consists of the improved atomizing method herein fullydescribed and particularly pointed out in the claims.

In carrying out the invention, the magnesium to be atomized is meltedand the requisite amounts of the two metals, zinc and zirconium, aredissolved in the melt. In the case of the zirconium enough is added toproduce in solution in the magnesium a concentration of 0.025 to 1 percent of zirconium by weight. A preferred concentration of zirconium isabout 0.05 to 0.6 per cent. In the case of the zinc enough is added tothe magnesium melt to produce therein a dissolved zinc concentration ofat least 0.25 per cent. A zinc concentration as high as 7 per cent maybe used. In general, a desirable proportion for the zinc is betweenabout 0.5 and 1.5 per cent of the melt by weight.

The magnesium melt containing the requisite concentrations of zinc andzirconium is brought to a temperature between about 680 and 800 C. andis then allowed to fall in a thin stream (e. g. 4; x in diameter) adistance of about 2 to 10 inches onto a spinning steel disc, either flator concave, the axis of rotation of the disc being substantiallyvertical. The point of impingement of the molten metal, which falls ontothe disc, is preferably at or close to the center of rotation of thedisc.

A concave disc having a spherical concavity is preferable. It isoriented so that the concave surface faces the falling stream of themolten zincand zirconium-containing magnesium. Discs having diameters of2 to 6 inches may be used. A preferable diameter of the disc is 3%inches. Running speeds of 2,000 to 100,000 R. P. M. or more may be useddepending upon the diameter and the strength of the steel.

The space in which the disc operates is charged with an inactive gas,such as natural gas or with one or more of the principal hydrocarbonconstituents thereof, e. g. methane, ethane, propane, butane, preferablyat room temperature, although temperatures up to about 240 C. may beused. The so-called inert gases may be used, c. g. helium, argon, andthese gases may be used at higher temperatures. Other gases unreactiveto magnesium may be used, e. g. hydrogen.

The temperature of the disc is important and is brought up to andmaintained at working temperature by the molten metal to be atomized.This is accomplished by suitably heating the molten metal beforeimpingement on the disc and allowing the so-heated metal to fall ontothe disc while it is rotating at working speed and continuing theimpingement until the disc thereby becomes heated. The disc is at leastpartially insulated against heat loss as by a backing of thermalinsulation to allow the working face to reach operating temperature. Theproper tem perature for the disc is readily ascertained by either ob--serving the Working surface or sampling the product thrown off as thedisc spins. When the disc is at a proper working temperature, it becomeswet with a fluid film of the molten metal which may be seen on visualinspection during operation. At the same time, when the disc is wet, themolten metal, which is thrown oil the disc and allowed to cool, is foundto be in fine uniformly sized spherical particles. The temperature thenof the molten magnesium, which is deposited upon the spinning disc andthere atomized in accordance with the invention, is made sufficientlyhot to heat the disc to and maintain it at a temperature above themelting point of the molten metal, so that by virtue of the presence insolution in the magnesium of the aforesaid amounts of zinc andzirconium, the disc becomes wetted by the molten metal. With a suitableamount of thermal insulation at the back of the disc, e. g. about inchthickness of asbestos paper, heating the molten metal to between about680 and 800 C. suifices to produce the desired molten metal film on thedisc. In this mode of operation, the disc remains smooth, splashing isvirtually eliminated with the consequent elimination of the formation ofirregularly shaped particles. The sieve analysis of the atomized productremains substantially constant during operation. The amount of extremelyfine or dust-like particles formed, if any, is negligible. Instead, themolten metal is atomized into a mass of spherical particles conformingto a relatively narrow range of desirable particle sizes.

The invention may be further explained and illustrated by reference tothe accompanying drawing showing an apparatus with which the methodinvention may be practiced.

In the said drawing wherein like numerals designate like arts: p Fig. 1is a diagrammatic view of the atomizing plant showing the elementsthereof and their relationship.

Fig. 2 is a detailed view largely in section of a portion of theapparatus of Fig. 1.

Fig. 3 is an enlarged sectional view on the line 3-3 of Fig. 2.

Referring to the drawing and more particularly Fig. 1, there is shown anatomizing tank 1 in which atomization takes place. The tank has aconical bottom 2 having an outlet 3 which is connected by pipe line 4 toa settling tank 5. The settling tank 5 has an outlet 6 at the bottom theopening through which is subject to control by a valve 7. The settlingtank is connected by a pipe 8 to the cyclone separator 9. The separatoris provided with a bottom outlet 10 controlled by a valve 11. The topoutlet 12 of the separator 9 is connected by a pipe 13 to a gas holder14 which holds a supply of inert gas which may be introduced through theinlet pipe 15 connected to a source not shown. Inert gas is withdrawnfrom the gas holder 14 through pipe 16 by means of the gas compressor17. The gas compressed by the compressor 17 is delivered through pipe 18to the cooler 19 in which the compressed gas is cooled. The cooledcompressed gas passes from the cooler through pipe 20 to the turbine 21shown in Fig. 2 in tank 1.

Referring more particularly to Fig. 2, turbine 21 is supported on legs22 the lower ends of which (not shown) are secured to the inside oftank 1. The turbine 21 drives the vertical spindle 23 to the upper endof which is welded face plate 24, as shown in detail in Fig. 3. As shownin the periphery of the face plate is cut a thread 25 which engages theinternally threaded recess 26 of the atomizing disc 27. The atomizingdisc 27 has a concave upper face 28. Clamped between the face plate 24and the back 29 of the disc 27 is a layer 30 of thermal insulation, suchas for example asbestos paper about Vs inch thick.

As shown in Fig. 2, the top 31 of tank 1 has a centrally disposedconcave portion 32 on which is mounted the furnace setting, indicatedgenerally by numeral 33. This is comprised of a relatively smallerdiameter lower portion 34 and an upper portion 35 having a largerdiameter so as to accommodate a melting pot 36. Attached to the bottomof the melting pot and forming an outlet therefor is the tubular spigot37 having an outlet 38 of smaller diameter than the bore 39 of thespigot. Between the outlet 38 and the bore 39 is a shoulder 40 whichforms a seat for a valve 41. As shown, the valve 41 is formed as ashoulder on the lower end of the push rod 42 which enters the bore 39 ofthe spigot. The push rod also carries a smaller diameter portion 43 as aprod capable of being pushed through the outlet 38 to clean it as thevalve 41 engages the shoulder 40 in closing the outlet 38.

The spigot has an external taper 44, the taper being designed to seat inthe internally tapered nipple 45, as shown, the bottom of which isjoined to the concave portion 32 of the tank 1 around the centralopening 46. The outlet 38 is directly above the center of the disc 27, adistance of about 2 to 10 inches. The furnace setting 33 is providedwith openings 47 and 48 in the lower and upper portions, respectively,through which the flame of heating burners (not shown) is projected toheat the tapered nipple 45, spigot 37, and melting pot 36 to a suitableoperating temperature. Telescope 49 is provided for viewing theatomizing disc.

In operation, when starting up, the opening 38 is closed by lowering thepush rod 42 so as to seat the valve 41 on shoulder 40 and a charge ofthe metal to be atomized is introduced into the melting pot 36. The potis maintained sufficiently hot to maintain the charge at about 680 to800 C. The compressor 17 is started up and inert gas is therebywithdrawn from the gas holder and compressed. The compressed gas isdischarged from the compressor through pipe 18 into the cooler 19 whichremoves more or less of the heat of compression. The so-cooledcompressed gas is delivered by pipe 20 to the turbine 21, therebyspinning the atomizing disc 27 The exhaust from the turbine enters thetank 1 by way of the turbine exhaust pipe 50 and maintains in the tankan inert gas atmosphere. When the disc 27 reaches a suitable speed ofrotation, e. g. 20,000 R. P. M., the push rod 42 is raised enough toallow the molten metal from pot 36 to fall in a thin stream through theopening 38 onto the concave surface 28 of the disc 27 while it spins.After a time, the disc 27 acquires a suitable operating temperature,viz. a temperature above the melting point of the metal to be atomized,the disc 27 on being so-heated becomes wetted with the magnesiumcontaining the zinc and zicronium so that the surface 28 is coated witha thin film 51 of the molten metal. Atomization then begins as themolten metal released from the pot on falling onto the molten metalcoated disc is flung off toward the sides of the tank 1 in tinyuniformly sized spherical drops which solidify into spherical particlesin the inert gas atmosphere of the tank 1. The atomized particles thusproduced fall onto the conical bottom 2 and are carried by the inertgas, exhausting from the turbine, through the outlet 3 into pipe 4 andthence into the settling tank 5. Most of the particles settle out of theinert gas in the tank 5 and may be withdrawn from time to time throughthe outlet 6 by opening valve 7. The relatively small amount of fineswhich do not settle out of the inert gas in tank 5 are carried into thecyclone separator 9 by the inert gas which flows from tank 5 into thecyclone through pipe 8. The inert gas separated from the fines in thecyclone separator is discharged through pipe 13 into the gas holder 14for recirculation through the system as described. From time to time,the fines may be withdrawn from the cyclone separator through outlet 10by opening valve 11.

As the operation proceeds, the tank 1 heats up to an extent dependingupon the temperature of the molten metal and rate of input as well asthe heat dissipating characteristics of the tank and the temperature ofthe incoming inert gas as well as the cooling eflect of the expansion ofthe inert gas in driving the turbine 21. In general, it is desirable tokeep the inert gas temperature in the atomizing zone, i. e. the areaadjacent to the spinning disc 27, below the temperature capable of beingwithstood by the gas used without decomposition. Temperatures in therange of 100 to 240 F. are generally satisfactory for hydrocarbon gases.

The following data of atomizing runs are illustrative of the effect ofzinc and zirconium on the quality of the atomized product and theireffect on the steel atomizing disc. In these runs, the disc wasmaintained above the melting point of the metal by the conditions ofoperation, the temperature of the molten metal being between about 680and 750 C. so that the disc was coated with a film 51 of molten metalexcept in the runs tabulated as blanks. In these runs, the disc was notwetted with molten metal. The atmosphere was natural gas for all thetabulated runs.

Table l.--At0mizing runs Metal Composition Disc Pounds Run N0. jgg ofCondition Percent Percent Percent Product Speed, Diameter, Zn Zr Mg R.P. M. Inches Before After 1 6 0. 6 Bal. 5 3, 083 Smooth. Smooth 9,000-9, 600 2. 2 3 0 6 Bai. 1. 75 (1 ....d0 12, 300-12, 500 2. 75 3 0.670. 064 Ba]. 5. 5 ..d0 7, 200-7, 500 3. 75 4 1. 53 0.25 Bal. 0.73 .do..7, 000 2.75 5. 0.85 0. 27 132.1. 1.0 (10.. 7, 000 2. 75 6. 1.36 0. 17Bal. 0.9 (10.. 7, 000 2. 75 7. 1.29 0. 09 B211. 0. 45 d0 7, 000 2. 75 8.0. 07 0. 05 Bal. 0. 75 d0 7, 000 2. 75 A Blank 4. 76 Eroded to depth 10,350-10, 450 4. 0

of 0.062 inch.

B Blank 0.76 0.011 Ba-l 0.78 hed 7,000 2. 75 0 Blank 0. 025 0. 28 Hal 1.2 48, 000-54, 000 2. 75 D Blank 0.92 Bal 1. 0 7, 000 3. 75

Table lL-Screen analysis of atomized product f i 'gg Total ScreenAnalysis Percent Run No. Start P @139 of duced MilllltGS +20 20/35 35/6555/100 ---1O[) 2 20. 5 9. 43. 40. U 6. 0 1. 5 1 90 925 10. 0 45. 0 36-75 5. 75 2. 5 240 2, 467 8. 5 43. 0 39. 0 6. 0 3. 5 300 3, 083 8. 5 41.U 40. 3 6. 5 3. 5 2 101 0. 25. 25 52. 25 15. 5 6. 75 105 1, 010 0. 5 20.0 53. 5 14. U 7. 0 90 900 0. 4 2G. 8 61. 9 8. 1 2. 8 3 125 1, 250 1. 025. 8 60. 2 9. 0 4. 0 240 2, 400 0. 4 26. 2 52. 2 8. 0 3. 2 330 3,320 1. 0 21. 6 64. 7 9. 3 3. 4

+35 10 39 G. 2 21.? 54. g g 24 2 7 5. 0 2. 5. 0 Blank e 45 445 7. 5 39.5 23. 4 29.6 55 544 9. 5 36. 8 20. 8 32. 9 72 692 21. 4 43. 9 18. 8 15.9

1 Corresponds to runs of Table I.

Referring to Table I, runs 1 to 8 inclusive, were carried out inaccordance with the invention by including 25 in the magnesiumsufiicient amounts of both zinc and zirconium to prevent erosion orattack by the molten metal on the atomizing steel disc, the condition ofwhich before and after the runs is tabulated. For comparison,

blanks were run with commercial electrolytic magnesium zirconium as inrun C, for example. The lack of uniformity of the particle size of theproduct of run C is shown in the screen analysis where the proportion ofthe product retained upon a No. 35 standard sieve progressivelyincreased during the run from 6.2 per cent to 21.4 per cent, while theproportion passing through a No. sieve and retained on a No. sievedecreased from 24 per cent to 18.8 per cent during the run. At the sametime, a large and varying amount of excessively fine material wasproduced as indicated by the figures in the -100 column of run C. screenanalyses for runs 1, 2, and 3, in which the content of zinc andzirconium was controlled in accordance In sharp contrast, the 50 withthe invention, show a uniformity of particle size throughout the runsand a negligible amount of fines below a No. 100 sieve size.

We claim:

1. In a method of atomizing magnesium the steps which consist indissolving in the molten magnesium from 0.025 to 1.0 per cent ofzirconium and from 0.25 to 7 per cent of zinc, letting fall theso-prepared molten metal in a thin stream upon a spinning disc of steelhaving a temperature above the melting point of the soprepared moltenmetal whereby the disc becomes wetted with a film of the molten metaland the molten metal is flung off the disc in fine uniformly sizedparticles, and solidifying and collecting the resulting particulatedmetal.

2. In a method according to claim 1 maintaining the disc in anatmosphere of hydrocarbon gas at a temperature below 240 C.

3. In a method according to claim 2 in which the molten metal is letfall at a temperature between 680 and 800 C.

References Cited in the file of this patent UNITED STATES PATENTS2,040,168 DeBats May 12, 1936 2,061,696 DeBats Nov. 24, 1936 FOREIGNPATENTS 510,320 Great Britain July 26, 1939

1. IN A METHOD OF ATOMIZING MAGNESIUM THE STEPS WHICH CONSIST INDISSOLVING IN THE MOLTEN MAGNESIUM FROM 0.025 TO 10 PER CENT OFZIRCONIUM AND FROM 0.25 TO 7 PER CENT OF ZINC, LETTING FALL THESO-PREPARED MOLTEN METAL IN A THIN STREAM UPON A SPINNING DISC OF STEELHAVING A TEMPERATURE ABOVE THE MELTING POINT OF THE SOPREPARED MOLTENMETAL WHEREBY THE DISC BECOMES WETTED WITH A FILM OF THE MOLTEN METALAND THE MOLTEN METAL IS FLUNG OFF THE DISC IN FINE UNIFORMLY SIZEDPARTICLES, AND SOLIDIFYING AND COLLECTING THE RESULTING PARTICULATEDMETAL.